The objective of the proposed research is to develop a novel class of fluorescent nanoparticles called "Polymer Dots" and test the feasibility of the nanoparticles for detection of single molecules in live cells using conventional fluorescence microscopy methods. Polymer dot nanoparticles are a promising new fluorescent nanoparticle technology based on fluorescent pi-conjugated polymers which provide substantial improvements in brightness as compared to conventional fluorescent dyes and nanoparticles. The specific aims of the project include optimizing and characterizing relevant figures of merit for single molecule detection, development and testing of bioconjugation strategies for targeting specific biomolecules within cells, and demonstration of single nanoparticle detection in living cells. A range of characterization techniques will be employed, including fluorescence spectroscopy, single molecule spectroscopy, atomic force microscopy, electrophoresis, and epifluorescence microscopy. Improvements in brightness and photostability of a factor of 100 to 10,000 as compared to conventional fluorescent dyes are expected, meeting the requirements for fluorescence-based single molecule detection in living cells. Demonstration of facile, flexible bioconjugation methods for targeting specific biomolecules of interest and detection of individual labeled biomolecules within living cells are also expected. The development of this novel fluorescent nanoparticle platform will facilitate future research involving monitoring transport and biochemical events of single molecules within living cells and could also provide the basis for the development of novel bioassays and biosensors.